The present invention relates to a method and an apparatus for observing an astronomical object. More particularly, the present invention relates to a realtime monitor of an astronomical object using speckle interferometry, which is suitable for observation of a double star.
FIG. 5 is a diagrammatic cross section of the stellar speckle camera installed at KPNO (Kitt Peak National Observatory). As shown, this camera comprises: shutters 1, 6 and 10; an image plane detecting mask 2; an objective 3; an atmospheric dispersion compensation prism 4; a wavelength filter 5; an image intensifier 7; a focusing lens 8; a camera 9; a film 11; and a telescope 12. A similar layout is adopted by other image detecting optical systems used in stellar speckle interferometry.
The layout of the optical system shown in FIG. 5 is such that the Cassegrain focus of the telescope 12 coincides with a position of the image plane detecting mask 2 which is just in front of the objective 3 and that the orientation of the optical system is in alignment with the optical axis of the telescope. The function of the image plane detecting mask 2 is to determine whether right focus has been attained. The image in the focus is relayed and magnified by the microscope objective 3. The magnified image is passed through the atmospheric dispersion compensation prism 4 for compensating for the prism effect of air atmosphere. In order to compensate for the image distortion introduced on account of the change in refractive index which occurs through strata of air atmosphere, the image is passed through the prism 4 whose refractive index is changed through layers. The wave-length filter 5 is a combination of a narrow-band (10 to several tens of nanometers) filter and an ND filter and is used to select proper wavelength and adjust the quantity of light. In order to magnify the image and accomplish rapid image detection using the narrow-band filter, the gain of light intensity must be much increased. To this end, a gain of the order of 10.sup.5 is attained with the image intensifier 7. In order to improve the SN ratio, several hundred images in a short period are consecutively recorded with the camera 9. When taking photographic records of image, the shutter speed is adjusted to be within the range of from 10 to several tens of milliseconds and each picture developed is subjected to the optical Fourier transform and the average power spectrum on the Fourier plane is determined by multiple exposure.
If a TV camera is substituted for the photographic recording, each picture is A/D-converted in 1/60 second and written into a frame buffer memory, followed by the Fourier transform with a computer to determine the average of the power spectra of several hundred pictures.
Another approach is illustrated in FIG. 6: an astronomical speckle image is passed through a lens 21 and focused on the photocathode of an image intensifier 22; the secondary electron image emitted from the image intensifier 22 is recorded as a charge pattern on an electro-optic crystal 23 and read out with a laser beam from a laser light source 24; the readout is subjected to the optical Fourier transform and multiple-exposed on a photographic film with a camera 27 to determine the average power spectrum (see Optical Engineering, vol. 17, No. 3, May-June 1978, pp. 261-263).
The system shown in FIG. 5 involves the need to perform the Fourier transform operation on as many as several hundred pictures by photographic processing. When the TV camera is used, the Fourier transform must also be performed and it takes a considerable time to determine the average power spectrum even if the computer is utilized. The system shown in FIG. 6 also depends on time-consuming photographic processing for determining the average power spectrum. Therefore, a common problem with these systems is their inability to accomplish realtime processing.